Implantable cardiac defibrillators (ICDs) are well known in the art. These devices, encapsulated in a conductive housing or enclosure, are generally implanted in the left pectoral region of a patient and electrically connected to the heart with one or more electrode-carrying leads. One lead includes at least one defibrillation electrode arranged to be positioned in the right ventricle. An arrhythmia detector detects ventricular arrhythmias, such as ventricular fibrillation. When such an arrhythmia is detected, a pulse generator delivers a defibrillating shock from the defibrillation electrode in the right ventricle to the conductive housing to terminate the arrhythmia. Alternatively, such arrhythmia terminating systems may further include another defibrillation electrode positioned in the right atrium and electrically connected to the conductive housing. In this arrangement, the defibrillating shock is delivered from the parallel connected right atrial electrode and the conductive housing to the right ventricular electrode.
Implantable atrial defibrillators are also known. These devices are also encapsulated in a conductive housing or enclosure and are electrically coupled to the heart by one or more electrode-carrying leads. The leads are known to include a defibrillation electrode positioned in the right atrium of the heart. When an arrhythmia detector detects an atrial arrhythmia, such as atrial fibrillation, an atrial defibrillating shock is then applied from the right atrial defibrillation electrode to the conductive housing.
Although not presently commercially available, combined implantable dual-chamber (atrial and ventricular) defibrillators continue to be investigated and under development. Because of the vast differences between ventricular and atrial arrhythmias, dual-chamber devices remain a challenge. For example, ventricular fibrillation is an immediately life threatening condition while atrial fibrillation, although uncomfortable and debilitating, is not a life threatening condition. Hence, there must be a preference for effective ventricular fibrillation treatment over atrial fibrillation treatment. Further, unless safety measures are taken in delivering atrial fibrillation terminating shocks, there is a potential for atrial fibrillation terminating shocks inducing ventricular fibrillation. Still further, by the time a ventricular fibrillation terminating shock is delivered, the patients are, in most occurrences, unconscious while, when atrial fibrillation terminating shocks are delivered, the patients are conscious and able to perceive discomfort from the atrial fibrillation termination shocks. Lastly, because of the relative locations of the atria and ventricles, the most effective ventricular fibrillation termination electrode configurations are different from the most effective atrial fibrillation termination electrode configurations.
Hence, in terms of arrhythmia termination, ventricular fibrillation termination electrode configurations must be those which provide the greatest assurance of successful arrhythmia termination. These configurations are those which exhibit the lowest ventricular defibrillation thresholds. In contrast, in arriving at an atrial fibrillation termination electrode configuration, consideration must be given to both atrial termination effectiveness and the degree of perceived discomfort by the patient to the delivered atrial arrhythmia termination shocks. The present invention addresses these issues.